1. Field of the Invention
This invention relates generally to control systems for dc traction motors and more particularly to systems for separately driving the armature and field circuits of a dc traction motor while optimizing energy efficiency.
2. Related History
DC electric motors have functioned as a motive source in various vehicles ranging from trains, to boats, automobiles, delivery vans, golf carts, trucks and fork lift vehicles. In many applications including automobiles, material handling trucks and fork lifts, the on-board stored energy supply comprised batteries which accounted for a significant proportion of the total vehicle weight. Depending upon the vehicle application, motor torque loads significantly varied and were affected by vehicle load variations, the incline of the travel path, (such as a loading ramp) and vehicle acceleration. It was common for vehicle pay-load variations to represent a change in the order of 33% of the total motor torque requirements.
In vehicles wherein the motive energy was furnished by an on-board battery system, the main motive element, commonly known as a traction system, included a series-wound dc motor coupled to one or more drive wheels through a reduction gear. Control of the direction of rotation of the motor was effected by controlling the polarity across the motor field-armature. Series-wound dc motors were limited to operation along their characteristic commutation curve and, as a result, variations in torque load resulted in variations of motor speed, hence vehicle speed.
Separately excited motor control systems have been devised for independently and variably exciting the armature and field windings and thus remove the constraint of operation only along the motor's commutation curve limit. It has been found impractical, however, to implement systems with separately excited and variable armature and field control in conjunction with series-wound dc motors because the field current levels of such motors required large and relatively expensive control systems, specifically the field control portion of the control systems.
The substitution of shunt-wound dc motors for reducing the size and cost of the control systems as opposed to series-wound dc motors has been attempted. Under conventional control, wherein the field excitation of a shunt wound dc motor was constantly applied, the shunt-wound dc motor was incapable of providing a high starting torque and could not serve as a replacement for a series-wound dc motor. With the use of a motor control system wherein the armature and field of a shunt-wound dc motor were separately excited and variably controlled, shunt-wound dc motors were capable of operating with the high starting torque characteristics of a series-wound dc motor.
Systems for controlling a separately excited shunt-wound dc motor though microprocessor-based independent pulse width modulated frequency control of an armature chopper and a field H-bridge were disclosed in U.S. Pat. No. 5,070,283 and U.S. Pat. No. 5,039,924, both of which were issued to the present applicant.
The inventions disclosed in the aforementioned patents generally comprised an armature voltage amplifier connected to the shunt-wound dc motor armature for varying the applied armature voltage and a field current amplifier for controlling the direction of armature rotation and for varying the voltage applied to the field winding. Rather than allowing the field current to follow armature status, a processor was employed to effect complete decoupling of torque and speed characteristics (patent 5,070,283). The processor received signals representative of a reference armature speed value and a reference armature current value and employed matrix algebra to transform such signals into filtered input references. A controller received signals representative of sensed armature current and sensed armature speed and generated conditioned motor output signals, utilizing matrix algebra. The difference between one of the filtered input reference signals and one of the conditioned motor output signals comprised a decoupling armature effort signal applied to the armature amplifier and the difference between the other filtered input reference and the other conditional motor output comprised a decoupling field effort signal applied to the field circuit. Both decoupling effort signals were generated for maintaining constant armature speed with varying torque loads.
In patent 5,039,924, motor efficiency was optimized with a processor which controlled the field current amplifier responsive to desired armature speed, sensed armature speed, sensed armature current and sensed field current. The processor functioned with a feedback controller which generated an optimal ratio as a function of the sensed armature speed and the sensed field current.
While the systems disclosed in these prior patents functioned to provide motor operation capable of producing variable torque while maintaining constant speed or functioning to optimize motor efficiency, such systems were subject to certain disadvantages in terms of cost and practical efficiency.
The systems disclosed in both of the aforementioned patents functioned independently of each other and required separate algorithm sets for different conditions and could not be incorporated for simultaneous operation in a single system.
Additionally, the prior optimization system was premised upon the unnecessary constraint that armature control must operate totally independent of field control and field control must operate totally independent of the armature control whereas practical implementation must recognize the status of the armature circuit for optimized field control and the status of the field circuit for armature control.
The operational constraints for totally independent control found in patents 5,039,924 and 5,070,283 required complex and unnecessary programing from a practical system operation standpoint and significant manufacturing costs in terms of initial programming expense and hardware configuration.
Further, the applicant realized that by driving only the armature circuit to maintain constant speed under varying torque loads and allowing the field current to follow the driven armature current, adequate decoupling of torque and speed characteristics could be attained which would suffice for practical implementations, without the added cost and complexity of separately driving the field circuit to achieve decoupling control.
The applicant herein has appreciated that interrelationships between armature and field conditions, such as, mutual inductance, mandate a control system wherein the field and armature are not driven totally independently of one another.
Further, the applicant herein recognized the need for a single control system capable of simultaneously providing practical levels of torque/speed decoupling and simultaneous optimization of motor efficiency.